ENH: interpolate: add the rescale argument to griddata and NearestNDInterpolator

When it was added to the linear and cubic nd interpolators, the
nearest-neighbor one and griddata() were missed.

Also, refactor NDInterpolatorBase to avoid some work that is unnecessary
in the nearest neighbor interpolator.
Merge pull request #2979 from pv/rescale-griddata

ENH: interpolate: add the rescale argument to griddata and NearestNDInterpolator